Conventionally, for amplification of an input voltage, operational amplifiers have been used in a variety of technical fields. For example, operational amplifiers have been used for a pipeline AD converter. The pipeline AD converter has a plurality of conversion stages each including an analog-digital conversion circuit, a digital-analog conversion circuit, a capacitor, an operational amplifier and the like. In the conversion stages, analog-digital conversion and amplification/output of the residual voltage are performed sequentially, whereby an analog signal is converted to a digital signal bit by bit. Such a pipeline AD converter is often used for video and communication purposes.
FIG. 14 is a circuit diagram of a general operational amplifier. The operational amplifier of FIG. 14 amplifies an input differential voltage composed of input voltages Vinp and Vinn and outputs the amplified voltage as an output differential voltage composed of output voltages Voutp and Voutn. The input voltages Vinp and Vinn are respectively supplied to the gates of differential transistors T901a and T901b. A bias voltage VBN is supplied to the gate of a current source transistor T902, and a bias voltage VBP is supplied to the gate electrodes of load transistors T903a and T903b. 
FIG. 15 shows a layout pattern of the operational amplifier of FIG. 14. The gate electrodes of the differential transistors T901a and T901b are electrically connected to an input node receiving the input voltage Vinp and an input node receiving the input voltage Vinn via interconnects W901a and W901b, respectively. The gate electrode of the current source transistor T902 is connected to a bias node receiving the bias voltage VBN via an interconnect W902. The gate electrodes of the load transistors T903a and T903b are connected to a bias node receiving the bias voltage VBP via an interconnect W903.
An operational amplifier is designed through various design steps (a device placement step, a wiring step, a rule verification step, etc.) so that the circuit performance (e.g., current drive capability) of the operational amplifier is in a desired level of performance. In the device placement step, placement of elements (transistors, etc.) constituting the operational amplifier is determined, and the resultant placement of elements is described in a layout pattern. In the wiring step, interconnects for connecting the elements is described in the layout pattern. In the rule verification step, whether or not an operational amplifier circuit can be fabricated normally according to the layout pattern (whether or not the layout pattern does not violate various design rules is verified).
Layout preparation methods for a pipeline AD converter are disclosed in Japanese Laid-Open Patent Publication No. 2002-223165 (Patent Document 1) and the like.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-223165 (p. 11, FIGS. 7 and 8)